1. Field of the Invention
The present invention concerns converting the wavelength of optical signals.
2. Description of the Prior Art
The wavelength of optical signals is advantageously used, especially in optical switching or transmission systems, for multiplexing, for example, to enable different optical signals conveyed by a common transmission medium to be distinguished from each other, or for routing or space-division switching purposes, to assign optical signals a particular propagation path from a set of possible propagation paths. The problem frequently arises in such systems of "changing" the wavelength of an optical carrier modulated with the data to be transmitted by these systems.
One way to change the wavelength is to detect said data by means of an optical receiver and then to use the electrical signal produced by this transducer to modulate an optical source, possibly a tunable optical source, emitting at the required wavelength.
Another method which avoids optical-electrical conversion followed by electrical-optical conversion has recently been made possible by a wavelength converter opto-electronic component consisting of a semiconductor laser diode using a tandem electrode structure with a saturable absorber. This laser can be triggered optically and tuned electrically.
A component of this kind is described, for example, in the document: Optical Fiber Communication Conference--New Orleans--Jan. 25, 28 1988: "An optical--wavelength conversion laser with tuneable range of 30 A" S. Yamakoshi, K. Kondo, M. Kuno.
Briefly, it comprises an optical waveguide formed of a series of juxtaposed sections one of which, into which the incident optical signal is injected, incorporates the saturable absorber. These sections are provided with respective control electrodes for applying various electrical control signals, namely in this instance a saturable absorber control signal, a gain control signal and a signal controlling the wavelength of the optical signal emitted by the laser.
The combination of the gain control signal (a current I.sub.1, for example) and the saturable absorber control signal (a current I.sub.2, for example) defines an operating point of the laser at which the laser may be triggered optically, according to the incident optical power. Moreover, when the laser is actually triggered, the wavelength of the optical signal that it emits is determined by the wavelength control signal.
An object of the present invention is to control an optical wavelength converter component of the type described above, in particular to generate automatically the control signal to be applied to the control electrodes of a component of this kind to obtain the required emission wavelength for a given component.
Depending on the intended application of the wavelength converter, said required emission wavelength may optionally be obtained from specific data included in the incident optical signal applied to the converter; one application where it can be obtained from such specific data is in a photonic switching matrix enabling switching of data in the form of fixed length asynchronous transfer mode cells, in which case said specific data is in the cell header. These cells have a fixed format comprising an information field preceded by a header containing control information.
The invention is particularly beneficial in this application to a photonic switching matrix.